Existing immunoassay methods involving measurement of binding rate suffer from the problem that the exact time when the reaction actually starts cannot be known because of practical difficulties in mixing. This limits accurate determination of initial binding rates. Thus typically a constant initial rate of reaction is required. For example, methods utilizing constant initial rate have been described in U.S. Pat. Nos. 4,205,954, 5,371,021 and 5,583,055. In another type of method, the peak binding rate is measured, for example as described in U.S. Pat. Nos. 4,157,871, 4,204,837, 4,268,171, 4,766,083 and 4,835,110.
A disadvantage of these methods is the relatively long time needed to gather enough information from the reaction in order to be able to determine the concentration of the sample. A further disadvantage is the need to start the actual measurement immediately after the addition of the sample.